This invention relates to flangeless winding cores for magnetic tape, adapted for stacking with their surfaces in contact and the tapes wound thereabout in supporting contact.
The recording of sound on magnetic tape and the packaging and marketing of such tape recordings has become a large-scale industry incorporating many mass production techniques. The blank tape is provided by the tape manufacturer in lengths of up to several thousand meters wound about a standardized flangeless winding core, such units of blank tape being known in the trade as "pancakes" which may have a diameter up to fifteen inches (38 cm) or more. The tapes are supplied in this form to cassette manufacturers and recording studios where they are recorded and packaged according to their particular purpose. The standard core, originally standardized for 1/4" (6.3 mm) tape, is a flangeless plastic ring having an axial height of 8.9 mm (0.350 in.) because the holding devices of the sophisticated and expensive machines used for processing the "pancake" units are mainly adjusted to this width. In view of the difference between the width of the winding surface of a core having this height and the width of the standard cassette tape, unless the core is designed to take this difference into account, it is necessary to insert rings of foam or cardboard between the stacked reels of tape for transport so that the individual reels will not be deformed by the jarring and jolting to which they are likely to be subjected in transport.
Two factors are of particular importance in the design of a winding core to eliminate the need for packing materials while at the same time ensuring their proper handling on existing processing machinery. First, the core should enable stacking of such cores with tape wound therearound, but without intervening support material between the tape on adjacent cores, to reduce bulk and cost of transporting and handling the cores and tape. Secondly, the expensive and sophisticated machinery utilized in processing the tape usually employs core holding devices adjusted to the standard 8.9 mm (0.350") core height, thus creating a significant design requirement. Briefly, to achieve the desired stacking efficiency, the core should have a height approximately equal to the width of the tape wound therearound, or 3.81 mm (0.150") for the tape used in compact cassettes. On the other hand, to permit processing of the "pancake" on existing expensive machinery, a core height at the centerhole greater than the width of standard 3.81 mm (0.150") and 6.3 mm (0.25") tapes is required; specifically, the height of the core at its standard 3-inch (7.62 cc) centerhole desirably should be 8.9 mm (0.350 inch) to conform to the current standard norm for existing processing machinery.
These somewhat contradictory requirements are addressed in Ender et al U.S. Pat. No. 4,081,151 by providing deformations which alternately project upwardly and downwardly from the median plane of an annular core body by half the total height of the core, the deformations on the two sides of the core being interlockable with each other when the cores are stacked so that the winding surfaces of adjacent cores are situated closely adjacent one another. These deformations have a rather complex undulating shape, and since the outwardly facing surfaces of the projections are inclined at an angle of about 60.degree., a core is not positively locked with an adjacent core stacked therewith and may lead to the inability of the stacked cores to withstand forces which might tend to rotate the cores relative to one another. In other words, because of the shallow locking angle between interlocking deformations, the cores are subject to being forced out of interlocking relationship and to be axially displaced from each other upon application of forces tending to cause relative rotation of stacked cores.
In another embodiment disclosed by Ender et al, the winding core has a ridge-valley configuration along the top and bottom of the core, and indentations along a ridge are provided to receive pins projecting from a mating valley to prevent relative rotation of adjacent cores. This embodiment has the disadvantage, however, that the core can only be stacked one way up; that is, the elevated side of one core of the stack must always be placed in the recessed surface of the other.
The problem of satisfying the stacking and processing requirements for tape winding cores is also addressed in Schor U.S. Pat. No. 4,201,353, by providing an annular body having an outer circumferential surface on which the tape is wound and a center hole around which a multiplicity of recesses are uniformly distributed. A set of six opposed projections extend upwardly and downwardly from the top and bottom surfaces of the annular body, the projections being arranged in a circular array adjacent the centerhole and spaced for mating with every third recess along the array of recesses of a sandwiching core. In the disclosed embodiment, the projections rise 2.46 mm (0.097") above the level of the top and bottom surfaces which, when added to a body thickness of 3.96 mm (0.156") gives a total height of 8.88 mm, substantially equal to the 9.9 mm (0.350") standard. In spite of its meeting this aspect of the standard, the commercial version of the Schor winding core tends to slip on the driving hub of existing processing machinery, probably because the overall height of the projections is 0.312", not the 0.350" disclosed in the patent, and thus not sufficient adequately to engage the hub.